1. Field of the Invention
The field of art to which this invention pertains is hydrocarbon processing and more specifically to a catalytic cracking process. More particularly the present invention is concerned with a method for regenerating a coke-contaminated cracking catalyst with the simultaneous carefully controlled combustion of CO to CO.sub.2 within a regeneration zone of a catalytic cracking process.
2. Prior Art
Regeneration techniques in which a coke-contaminated catalyst is regenerated in a regeneration zone occupy a large segment of the chemical arts. Particularly common are regeneration techniques used to regenerate a coke-contaminated fluidizable catalytic cracking catalyst within the regeneration zone of a fluid catalytic cracking (FCC) process. Until recent years the prior art has been primarily concerned with removing the maximum amount of coke from spent catalyst and at the same time preventing excessive temperature levels resulting from the conversion of carbon monoxide to carbon dioxide within certain portions of the regeneration zone, especially in the dilute-phase catalyst region where there is little catalyst present to absorb the heat of reaction and where heat damage to cyclones or other separation equipment can therefore result. Essentially complete CO conversion in conventional regeneration zones was prevented quite simply by limiting the amount of fresh regeneration gas passing into the regeneration zone. Without sufficient oxygen present to support the oxidation of CO to CO.sub.2, afterburning simply cannot occur no matter what the temperatures in the regeneration zone. As well, temperatures in the regeneration zone were generally limited to less than about 1250.degree. F. by selecting hydrocarbon-reaction-zone operating conditions of fresh feed streams or recycle streams or combinations thereof to limit the amount of coke on spent catalyst and hence the amount of fuel burned in the regeneration zone. The flue gas produced, containing several volume percent CO, was either vented directly to the atmosphere or used as fuel in a CO boiler located downstream of the regeneration zone. Usual FCC startup practice, familiar to those skilled in the art of FCC processes, was to initially manually regulate the flow of fresh regeneration gas to the regeneration zone in an amount insufficient to sustain essentially complete CO conversion while at the same time limiting regeneration zone temperatures to a maximum of about 1250.degree. F. When reasonably steady-state control of the FCC process was achieved the flow rate of fresh regeneration gas was then typically regulated by instrument control directly responsive to a small temperature differential between the flue gas outlet temperature (or the dilute phase disengaging space temperature) and the dense bed temperature to maintain automatically this proper flow rate of fresh regeneration gas to preclude essentially complete conversion of CO to CO.sub.2 anywhere within the regeneration zone. As the temperature difference increased beyond some predetermined temperature difference, indicating that more conversion of CO was taking place in the dilute phase, the amount of fresh regeneration gas was decreased to preclude essentially complete conversion of CO to CO.sub.2. This method of control is exemplified by Pohlenz U.S. Pat. Nos. 3,161,583 and 3,206,393. While such method produces a small amount of O.sub.2 in the flue gas, generally in the range of 0.1 to 1 vol.% O.sub.2, it precludes essentially complete conversion of CO to CO.sub.2 within the regeneration zone.
Until the advent of zeolite-containing catalysts, there was little economic incentive for essentially complete conversion of CO to CO.sub.2 within the regeneration zone. The use of the zeolite-containing FCC catalysts, which are more stable thermally and which have lower coke-producing tendencies, and the use of higher hydrocarbon conversion zone temperatures, however, often made additional heat input into the FCC process desirable. Typically additional heat was provided by burning external fuel such as torch oil in the regeneration zone or by adding or increasing the amount of feed preheat in external feed preheaters. Thus heat was typically being added to and then later removed from the FCC process by two external installations, a feed preheater and a CO boiler, each representing a substantial capital investment. Catalyst regeneration processes described in the recent prior art have recognized the advantages of essentially completely converting CO to CO.sub.2 and recovering at least a portion of the heat of combustion of CO both within the regeneration zone. Examples of such regeneration processes are Stine et al U.S. Pat. No. 3,844,973 and Horecky, Jr. et al U.S. Pat. No. 3,909,392. The advantages of such processes are now well known; such regeneration processes permit the reduction or elimination of feed preheat, the elimination of CO air pollution without the need for external CO boilers, and, when coupled with hydrocarbon-reaction zones of modern design, improved yields of more valuable products.
Regeneration processes employing CO conversion promoters or catalysts are not novel; indeed prior art processes for regenerating fluidizable coke-contaminated cracking catalysts have employed such promoters or catalysts. For example, in the fluid catalytic cracking process described in Kassel U.S. Pat. No. 2,436,927, which issued in 1948, a physical mixture of discrete particles of a cracking catalyst and discrete particles of a supported CO oxidizing catalyst is employed in a dense-phase region of a regeneration zone to enhance CO conversion in the dense phase thus preventing "afterburning" in the dilute-phase region of the regeneration zone. In the process described in Chen U.S. Pat. No. 3,364,136, which issued in 1968, a mixture of a cracking catalyst and a shape-selective crystalline aluminosilicate containing oxidation catalyst within its internal pore structure is used to control the CO.sub.2 to CO ratio in the regeneration zone without influencing the reaction taking place in the hydrocarbon reaction zone. In the process described in Wilson U.S. Pat. No. 3,808,121 two separate catalysts of different particle size and composition are employed; a cracking catalyst and a CO oxidation catalyst preferably supported in a matrix material such as alumina spheres and monoliths. Moreover, the supported CO oxidation catalyst is confined within the regeneration zone and does not pass out of that zone to the hydrocarbon reaction zone as does the cracking catalyst. Coke and CO are oxidizing in the regeneration zone to minimize CO in the flue gas.
Thus the prior art regeneration processes have employed CO oxidation promoters in one of two ways: (1) on discrete particles of a matrix or a support, particles to be mixed with the fluid cracking catalyst, and (2) as a part of component of the fluid cracking catalyst itself. Mixtures of cracking catalyst and supported CO oxidation promoter tend to be non-uniform which can result in a CO concentration in the flue gas in excess of allowable emission limitations. The use of a cracking catalyst containing as a component some predetermined concentration of a CO oxidation promoter makes it difficult to achieve in any particular regeneration zone the optimum concentration of oxidation promoter suitable for the operating characteristics of that particular regeneration zone or required to achieve a particular change in a dependent process variable.
By the method of our invention a CO oxidation promoter is added to the regeneration zone independently of the cracking catalyst, coke from spent catalyst is oxidized to produce regenerated catalyst and, essentially simultaneously, CO is converted to CO.sub.2 in the presence of the promoter and regenerated catalyst within the regeneration zone. A carbon monoxide oxidation promoter can be easily and precisely added to a regeneration zone, particularly when it is in a liquid, in amounts to control the CO concentration in the flue gas, to control a temperature within the regeneration zone or to control the amount of residual carbon on regenerated catalyst. Addition of a CO oxidation promoter by the method of our invention is therefore more economical than either employing separate particles of supported CO oxidation promoter along with a cracking catalyst or employing a cracking catalyst to which has been added a CO conversion promoter during the catalyst manufacturing procedure. Additionally the method of our invention gives to the refiner as an operating variable what heretofor had been essentially a fixed operating condition. Thus with the method of our invention the refiner has increased operating flexibility. Our method is applicable to any fluid catalytic cracking process, existing or new.